1. Field of the Invention:
The present invention relates to a process for producing a fluorinated copolymer having ion-exchange groups. More particularly, it relates to a process for producing a fluorinated copolymer having carboxylic acid type cation exchange groups which is effectively used as an ion exchange membrane in an electrolysis of an aqueous solution of electrolytes, by a copolymerization in an aqueous medium.
2. Description of Prior Art:
It has been known to produce sodium hydroxide in a cathode compartment by partitioning the anode compartment and the cathode compartment and feeding an aqueous solution of sodium chloride etc. into the anode compartment to electrolyze it by a diaphragm electrolysis.
Recently, it has been proposed to use a cation exchange membrane which does not substantially pass the electrolyte and selectively passes alkali metal ions instead of an asbestos diaphragm.
It has been especially proposed to use a fluorinated cation exchange membrane having high alkali resistance and chlorine resistance.
For example, it has been known to use cation exchange membranes of fluorinated polymers having carboxylic acid groups or sulfonic acid groups as the ion exchange group. (Japanese Unexamined Patent Publication Nos. 37395/1973 and 120492/1975).
It has been found that when the cation exchange membrane of the carboxylic acid type fluorinated copolymer is used in a diaphragm electrolysis of an aqueous solution of alkali metal chloride, an alkali metal hydroxide having high purity can be obtained and the operation in high current efficiency and high current density can be attained and high concentration of an alkali metal hydroxide can be obtained in the cathode compartment.
The excellent characteristic has been found that higher than 90% of the current efficiency can be given even though the concentration of sodium hydroxide is higher than 40%.
However, there has not been found any proposal to advantageously produce the fluorinated polymers having carboxylic acid type cation exchange groups.
The inventors have found that in order to attain the above-mentioned high electrolytic characteristics, it is important to provide an ion exchange capacity of the fluorinated polymer having ion-exchange groups in a range of 0.5 to 4 meq/g dry polymer, preferably 0.8 to 3 meq/g dry polymer, especially 1.0 to 2.2 meq/g dry polymer (meq = milli-equivalent) and to provide higher molecular weight.
When the ion exchange capacity is increased, the molecular weight of the fluorinated polymer has been lowered whereby the mechanical strength (such as ultimate tensile strength and ultimate elongation) of a membrane thereof is lowered.
The inventors have studied, given the above-mentioned knowledge, to provide an advantageous process for producing a fluorinated polymer having carboxylic acid type cation exchange groups especially a fluorinated polymer having high molecular weight and high ion exchange capacity.
As the result, it has been found that a desired fluorinated polymer can be obtained by copolymerizing a fluorinated olefin and a fluorinated monomer having carboxylic acid type functional group in an aqueous medium under specific conditions.